1. Field of the Invention
The present invention relates to a plating apparatus and a method of managing a plating liquid composition. More particularly, the invention relates to a plating apparatus for forming an interconnection by embedding copper in a fine interconnection groove defined in a surface of a semiconductor substrate through copper sulfate plating, and a method of managing an additive in a plating liquid used in such a plating apparatus.
2. Description of the Related Art
Processes of depositing a metal film by plating have widely been used in electronic industries for manufacturing a printed circuit board and the like. Recently, there have been made more attempts to use copper (Cu) as a metal material for forming interconnection circuits on semiconductor substrates because of its low electric resistivity and high resistance to electromigration, instead of aluminum or aluminum alloy. Copper interconnections are generally formed by embedding copper in fine recesses defined in a surface of a semiconductor substrate. Processes for forming copper interconnections include a chemical vapor deposition (CVD) process, a sputtering process, and a plating process. In any of these processes, a copper film is deposited on the entire surface of the semiconductor substrate, and then unwanted deposited copper is removed from the semiconductor substrate by chemical mechanical polishing (CMP).
FIGS. 1A through 1C show an example of a process for forming a copper interconnection on a substrate W through copper plating. As shown in FIG. 1A, an oxide film 2 of SiO2 is deposited on an electrically conductive layer 1a on a semiconductor base 1 on which a semiconductor device has been formed. A contact hole 3 and an interconnection groove 4 are formed in the oxide film 2 by a lithography etching technology. Then, a barrier layer 5 made of TaN or the like is formed on the oxide film 2, and a seed layer 7, which is used as a feeding layer in an electrolytic plating, is formed on the barrier layer 5.
Subsequently, as shown in FIG. 1B, the surface of the substrate W is plated with copper to fill the contact hole 3 and the interconnection groove 4 with copper and to form a copper film 6 on the oxide film 2. Thereafter, the surface of the substrate W is polished to remove the copper film 6 from the oxide film 2 so that the surface of the copper film 6 filled in the contact hole 3 and the interconnection groove 4 is made substantially even with the surface of the oxide film 2. Thus, as shown in FIG. 1C, an interconnection comprising the copper film 6 is formed.
The seed layer 7 is generally formed by sputtering or CVD. The copper film 6 is formed by an electrolytic copper plating process using a plating liquid which generally comprises a copper sulfate plating liquid including copper sulfate and sulfuric acid.
As circuit interconnections become finer, interconnection grooves or plugs have higher aspect ratios. In view of such tendencies, a strict technical requirement has been imposed for completely embedding copper in fine recesses, 0.1 xcexcm or less wide, defined in substrates, without causing defects.
To meet such a requirement, it is necessary to optimize a geometrical shape of an electrolytic tank and operating conditions such as electric conditions in the plating process. However, it is most important to completely manage the composition of the plating liquid. The plating liquid comprises main components including metal ions to be deposited and counter ions thereof, and additives for locally controlling the rate of the deposition reaction to equalize the deposition of the metal ions on a surface to be plated. Although the amount of additives added is extremely small, the additives have a highly significant effect on the plating process. Therefore, the plating process may disadvantageously be affected by the additives unless the additives are strictly managed in their concentrations.
In recent years, it has been required to minimize loads on the environment in various production processes. Particularly, it is expected to use a plating bath for a longer time in the plating process, which is highly likely to produce a waste liquid having a high concentration. From these points of view, it is desirable to positively manage the concentration of the plating liquid for efficiently using the plating liquid.
Heretofore, however, almost no efforts have been made to manage plating liquids. In a conventional plating process, certain amounts of plating liquid components are added to the plating liquid at given intervals of time for replenishment, and a plating bath is replaced as a whole after the plating liquid components have been added a certain number of times.
In an electric plating process, the state of the deposition reaction is monitored based on electrochemical measurement such as cyclic voltammetric stripping (CVS), and information on the concentrations of additives is obtained indirectly from the measured results. In this CVS process, a cyclic voltammogram is plotted with use of a potentiostat and a potential scanner, and information on the concentrations of the additives and contaminants in the plating liquid is obtained indirectly from the shape of the plotted cyclic voltammogram. However, since the CVS process is based on an indirect approach, it cannot meet requirements for management of the plating liquid with higher accuracy.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a plating apparatus which directly separates and quantifies an additive in a plating liquid to manage a composition of the plating liquid with high accuracy, and a method of managing a composition of a plating liquid in such a plating apparatus.
According to a first aspect of the present invention, there is provided a plating apparatus comprising a plating unit having a plating bath for holding a plating liquid therein; a plating liquid monitoring unit having a liquid chromatography device for separating and quantifying an additive in a sample of the plating liquid, and having an arithmetical unit for comparing a quantified value of the additive with a given concentration predetermined for the additive and for producing an output signal representing the compared result; and an additive replenishing unit for adding a solution including the additive from an additive tank to the plating liquid in the plating bath based on the output signal from the arithmetical unit in the plating liquid monitoring unit.
With the above arrangement, the additive in the plating liquid is directly separated and analyzed (quantified) with the liquid chromatography device, and additive which is insufficient or expected to be insufficient is properly added to the plating liquid. Thus, variations in the amount of the additive in the plating liquid are kept within a certain range.
According to a preferred aspect of the present invention, the liquid chromatography device comprises an evaporative light-scattering detector for quantifying the additive. The evaporative light-scattering detector can detect the intensity of light scattered by the solute that remains unevaporated after the sample has been evaporated through spraying. Thus, the evaporative light-scattering detector can basically detect any substances and has sufficient detecting sensitivity at practical concentration levels of additives. Therefore, the evaporative light-scattering detector can simplify the entire system. In the case where the evaporative light-scattering detector has insufficient detection sensitivity for some reasons, the additives may be concentrated to a detectable level by way of pre-column concentration.
According to another preferred aspect of the present invention, the plating liquid comprises a sulfuric acid copper plating liquid for embedding copper in a fine recess defined in a substrate to form an interconnection. Ionic components are removed from the plating liquid before the additive is quantified. With this arrangement, the main components such as sulfuric acid ions, copper ions, and chlorine ions, which are present at extraordinarily high concentrations, are removed in advance. Accordingly, a trace amount of additive in the plating liquid can easily be detected.
According to still another preferred aspect of the present invention, the additive comprises at least one of an oxygen-containing water-soluble polymeric compound, a sulfur-containing organic compound, and a nitrogen-containing organic compound. The oxygen-containing water-soluble polymeric compound may comprise polyethylene glycol, polypropylene glycol, or the like. The sulfur-containing organic compound may comprise disulfide or the like. The nitrogen-containing organic compound may comprise polyamine or the like.
According to a second aspect of the present invention, there is provided a method of managing a plating liquid composition, comprising: sampling a plating liquid in a plating bath; separating and quantifying an additive in the sampled plating liquid with liquid chromatography; comparing a quantified value of the additive with a given concentration predetermined for the additive; and adding a solution including the additive to the plating liquid based on the compared result.
According to a preferred aspect of the present invention, ionic components are removed from the plating liquid in advance before the additive is quantified.